1. Field of the Invention
The invention relates to electrochemical cells and more particularly to fuel cells which use alkali metal as the anodic reactant.
2. Description of Prior Art
Generally, batteries or electrochemical cells are sealed devices which contain fixed amounts of cathodic and anodic reactants, which are usually quite heavy. If the battery is a "primary" device it must be replaced after one use. For remote, especially extraterrestrial, sites, the difficulty and expense of resupply can be burdensome.
Electrochemical cells which use an alkali metal as anodic reactant have been described. U.S. Pat. Nos. 3,932,195 to Evans et al., 3,982,9589 to Partridge et al. and 4,041,216 to Desplanches et al. teach batteries wherein the alkali metal, sodium, is the anodic reactant and sulfur is the cathodic reactant.
The use of other alkali earth metals as the anodic reactant in fuel cells is also known. Fuel cells are electrochemical cells wherein one or both of the chemicals supplying the electrodes are stored separately and provided to electrodes on demand. Such fuel cells are particularly useful in environments where the replacement of electrochemical cells which have used up their reactant chemicals is especially inconvenient. See, for example U.S. Pat. No. 4,491,629 to Sarbacher et al. The '629 patent teaches a battery wherein aluminum is the anodic reactant. It is suggested that lithium would also be a suitable anodic reactant due to its high energy. This battery uses oxygen as the cathodic reactant and its use is therefore restricted to locations where oxygen is in large supply, limiting its use to terrestrial locations.
Since the electrolytes used in many types of fuel cells are materials which are easily reactive with carbon dioxide, the entrance of carbon dioxide into these cells is therefore undesirable. The carbon dioxide reacts with the electrolyte and forms a precipitate which accumulates at the surface of the porous electrodes of the cell, reducing the efficiency and performance of the cell. U.S. Pat. No. 3,511,712 to Giner teaches a process wherein undesirable carbon dioxide is removed from gas streams of oxygen and hydrogen which are supplied to the cathode or anode of a fuel cell.
U.S. Pat. No. 4,041,210 to Van Dine teaches a power plant which uses fuel cells wherein hydrogen is provided to the anode, oxygen along with carbon dioxide is provided to the cathode, and an alkali metal carbonate is the electrolyte. U.S. Pat. No. 4,317,865 to Trocciola et al. and U.S. Pat. No. 4,581,302 to Vine et al. teach an electrolyte matrix for a fuel cell which uses carbonates which are molten at operating such as alkali metal carbonates, as the electrolyte. U.S. Pat. No. 4,591,538 to Kunz teaches the use of lithium carbonate and potassium carbonate as the electrolyte in a fuel cell.
Fuel cells having alkali metal carbonates as electrolytes are thus known. These fuel cells, however, require the use of oxygen as the cathodic reactant. Although carbon dioxide is provided to the cathode along with the oxygen, it is the oxygen which is reduced at the electrode. Carbon dioxide acts as a depolarizer, reacting with the reduced oxygen to remove it from the electrode.
Therefore, while several of the above patents are directed to fuel cells wherein carbon dioxide is used in addition to oxygen at the cathode, there remains a need for electrochemical cells which use carbon dioxide exclusively at the cathode for use in environments where carbon dioxide is abundant and oxygen is absent.